NAS (novel aluminosilicates) as catalysts for the aromatisation of propane: Studies of zinc and gallium modified zeolite-based systems having various extents of XRD crystallinity

A review of existing literature on the behaviour of H-, Zn- and Ga-ZSM-5 systems in the aromatisation of propane suggest that no clear picture emerges regarding: (i) the relative merits of zinc and gallium with respect to activity and aromatics selectivity; (ii) the effects of the extent of crystallinity of the zeolitic material on catalysis. The work now reported involves a direct comparative study of H-NAS, Zn-NAS and Ga-NAS catalysts (NAS=novel aluminosilicates, comprising ZSM-5-based structures having XRD crystallinities ranging from the substantially amorphous to the partially crystalline and their highly crystalline ZSM-5 analogues. Zinc or gallium has been introduced into the parent zeolitic structure by means of solid-state ion-exchange. Zinc appears to enhance both activity and BTX (benzene, toluene and xylenes) selectivity, relative to the zeolitic parent solid, in contrast to gallium, which affects only BTX selectivity. In addition, optimum activity plus BTX selectivity under the conditions used are found for XRD crystallinities in the range 50–85%. The maximum BTX yields (single pass basis) are lowest for the H-NAS samples, markedly higher for the Zn-NAS catalysts, and at an intermediate level for the Ga-NAS samples.